car not not starting after changing the engine and its harness, gearbox and computer
 

I recently decided to change the engine, its harness, transmission and the computer for my honda fit almas year 2004 The reason behind this is that i have been struggling to fix my ever high rpm while driving at low speeds. However i did an upgrade because my cars engine was 1300cc but i decided to upgrade to a 1500cc engine but with the same type of transmission i was using before. After fixing everything now my mechanic tells me the car is not starting. Also when its on, the engine light appears and disappears which is not normal as it should disappear only when the car is started. What could be the problem?